(a) Field of the Invention
The present invention relates to a continuously variable transmission, particularly to a continuously variable transmission incorporating a belt (or chain) driven continuously variable transmission unit, and more specifically to a continuously variable transmission with the range of torque ratio expanded by the use of means such as planetary gear units.
(b) Description of the Prior Art
Due to rising demands for improvement in fuel economy in recent years, more attention is being paid to the use of continuously variable transmissions incorporating belt driven continuously variable transmission units as transmissions for motor vehicles.
Such continuously variable transmission generally includes a belt driven continuously variable transmission unit, a fluid coupling (or electromagnetically operated powder clutch), a reversing unit, a reduction gear unit and a differential gear unit. The range of torque ratio for such continuously variable transmission is limited due to restrictions imposed by availabe space and the minimum allowable radius of belt curvature. The torque range obtained from such continuously variable transmission alone is therefore insufficient to meet the demands for fuel economy and transmission performance on motor vehicles.
A continuously variable transmission has hence been proposed in which a planetrary gear unit is incorporated to expand the range of torque ratio, as disclosed in the unexamined publication of Japanese patent application (JP, A) 59-110954 (1984).
As illustrated in FIG. 23, the continuously variable transmission mentioned above includes a planetary gear unit 2 whose carrier C is connected to the output 5 of a belt driven continuously variable transmission unit 3, while either one of the ring gear R or the sun gear S (for example the sun gear) is connected to the input shaft 6 through a transfer unit 8 and the other (for example the ring gear) is connected to an output member such as a differential gear unit, with a clutch 10 and brake 11 provided between the input shaft 6 and the input 9 to the continuously variable transmission unit 3.
When the vehicle is running in forward direction, the rotation of input shaft 6 is converted under a designated torque ratio in the continuously variable transmission unit and transmitted through the clutch 10 to the carrier C of the planetary gear unit 2, and at the same time transmitted directly to the sun gear S through the transfer unit 8. The rotation of carrier C is thus amplified and delivered to the output member 7 from ring gear R. On the other hand, when the vehicle is running in reverse direction, brake 11 is applied and clutch 10 is disengaged, so that the rotation of the input shaft 6 is transmitted solely through the transfer unit 8 to sun gear S, to cause reverse rotation of ring gear R in cooperation with the carrier C which is in still condition.
In the continuously variable transmission incorporating a planetary gear unit 2 as described above, the combined sum of torques acting on the carrier C and sun gear S is delivered from the ring gear R. As the number of teeth on the ring gear R is normally quite large in relation to that on the sun gear S, a torque loop is formed, in which the torque acting on the carrier C is transmitted back to the input shaft 6 through the sun gear S and the transfer unit 8, and to the continuously variable transmission unit 3. A negative torque is hence transmitted to the sun gear through the transfer unit 8, and as a result the continuously variable transmission unit 3 has to convey a torque larger than that finally transmitted to the output member 7 in order to compensate for this negative torque. For example, if the ratio .rho.(Zs/Zr) between the numbers of teeth on the sun gear (Zs) and ring gear (Zr) is 0.5, from EQU Tc+Tr+Ts=O
and EQU Ts=.rho.Tr
(Tc=torque on carrier, Tr=torque on ring gear, Ts=torque on sun gear)
we obtain EQU Tc=Tr+Ts EQU =(1+.rho.)Tr EQU =1.5Tr.
In other words, the torque Tc on the carrier, which is the torque transmitted from the input shaft 6 through the continuously variable transmission unit 3, is 1.5 times the torque Tr on the ring gear, or the output torque transmitted to the output member 7.
Therefore, while it is possible to expand the range of torque ratio with this continuously variable transmission of prior art, there occurs the problem of the belt driven continuously variable transmission unit being subjected to a significantly larger torque.
As a result, excessive torque is exerted on the belt driven continuously variable transmission unit which, due to its dependence on friction for power transmission, is the least structurally reliable element among the entire continuously variable transmission system. Combined with the requirement for large side force on the pulleys to transmit the torque, the durability and efficiency of the continuously variable transmission could be impaired.